This invention relates to foundry binder compositions that contain a divalent sulfur compound, and/or foundry mixes where the aggregate contains a divalent sulfur compound. The foundry binders and foundry mixes are used to make foundry shapes, e.g. molds and cores. The presence of sulfur in the foundry shape facilitates the removal of foundry shapes (particularly internal cores) from the metal casting which is made by pouring molten metal into a casting assembly in which the foundry shapes are arranged. The invention also relates to a method of preparing foundry shapes, the shapes prepared, a method of making a metal casting, and metal castings by this process.
In the foundry industry, one of the procedures used for making metal parts is xe2x80x9csand castingxe2x80x9d. In sand casting, disposable molds and cores are fabricated with a mixture of sand and an organic or inorganic binder. The foundry shapes are arranged in casting assembly, which results in a cavity through which molten metal will be poured. After the molten metal is poured into the assembly of molds and cores and cools, the metal part formed by the process is removed from the assembly. The binder is needed so the molds and cores will not disintegrate when they come into contact with the molten metal.
Two of the prominent fabrication processes used in sand casting are the no-bake and the cold-box processes. In the no-bake process, a liquid curing catalyst is mixed with an aggregate and binder to form a foundry mix before shaping the mixture in a pattern. The foundry mix is shaped by putting it into a pattern and allowing it to cure until it is self-supporting and can be handled. In the cold-box process, a gaseous curing catalyst is passed through a shaped mixture (usually in a corebox) of the aggregate and binder to cure the mixture.
A binder commonly used in the cold-box fabrication process is a phenolic urethane binder. The phenolic urethane binder is mixed with an aggregate to form a foundry mix. The foundry mix is blown into pattern, typically a corebox, where it is cured by passing a gaseous tertiary amine catalyst through it. The phenolic urethane binder consists of a phenolic resin component and polyisocyanate component. Phenolic urethane binders are widely used in the foundry industry to bond the sand cores used in casting iron and aluminum. An example of a commonly used phenolic-urethane binder used in the cold-box process is disclosed in U.S. Pat. No. 3,409,575.
One of the problems with the using organic binders to form foundry shapes is that they can be too effective in binding the aggregate together. The result is that the foundry shapes are not readily separated from the metal part formed during the casting process. Consequently, time consuming and labor intensive means must be utilized to break down the binder so the metal part can be removed from the casting assembly. This is particularly a problem with internal cores, which are imbedded in the casting assembly and not easily removed.
The phenolic urethane cold-box process can be used to make cores and molds for the casting of ferrous and non-ferrous metal parts. Since iron castings are manufactured at about 1500xc2x0 C., any phenolic urethane binder used in making foundry shapes, i.e. internal cores, will undergo rapid thermal decomposition at this temperature. Because of this, the internal core can be easily separated from the iron casting. This does not occur when aluminum parts are cast because aluminum castings are manufactured at about 700xc2x0 C. At this lower temperature, the phenolic urethane binder does not readily decompose when the aluminum is cast, thus making complete removal of an internal core difficult.
Since light alloy casting, such as aluminum casting, is becoming increasingly used in place of iron as a means of reducing the weight of vehicle components such as engine blocks and manifolds, there is a need for developing new methods which facilitate the removal of internal cores. One method of facilitating removal of an internal core from a large aluminum casting (e.g. an engine block) or a complex aluminum casting (e.g. a water pump housing), is by baking the casting in a forced air oven at a high temperature for five to ten hours until the binder slowly decomposes. This procedure reduces productivity and requires forced air ovens and large amounts of energy. Alternatively, some aluminum castings can be violently shaken until the internal core is released. This procedure is inefficient and also reduces productivity.
Alternatives to phenolic-urethane cold-box binder systems can be used for aluminum casting that afford good core removal. For example, furan cold-box resins display excellent core removal characteristics in aluminum casting. However, furan resin binders build a tar like residue on tooling. This requires frequent cleaning, higher tooling costs, and lowers foundry productivity. See Richardson U.S. Pat. No. 3,879,339.
It is also known to use additives to improve the core removal characteristics of phenolic binders. For example, Ohashi et al, U.S. Pat. No. 4,600,733, teaches that a xe2x80x9cdisintegration assistantxe2x80x9d composed mainly of calcium hydroxide, calcium carbonate, barium hydroxide and/or barium carbonate will promote heat deterioration of condensation resins such as phenolic shell resins. Another example is described in Japanese Abstract J.P. 60,064,744, which discloses that metal bromides promote the collapsibility phenolic shell resins. S.U. Abstract 1,316,741 discloses the use of a water glass binder in mixture with iron oxide, sulfur, and silica based refractory filler that has improved knockout. The iron oxide is added to breakdown the binder which improves the knockout capability of the mixture.
This invention relates to foundry binder compositions comprising as a mixture:
(a) an organic foundry binder having at least one component; and
(b) an effective amount of a divalent sulfur compound where said divalent sulfur compound is present in at least one component of the binder.
An effective amount of divalent sulfur compound is an amount sufficient to facilitate removal of a foundry shape from a metal casting, where the foundry shape is made from the organic binder containing the divalent sulfur compound.
The invention also relates to foundry binder systems where (a) and (b) are separate components, and foundry mixes that contain a divalent sulfur compound in the foundry aggregate. The invention also relates to a method of preparing a foundry shape, the shapes prepared, a method of making a metal casting, and metal castings prepared by this process.
The presence of sulfur in the binder composition and/or the aggregate facilitates the separation of the foundry shapes (cores and molds) from the metal part made by pouring molten metal into the foundry assembly. The effect of the sulfur in the foundry mix is particularly noteworthy when making aluminum parts with a casting assembly having internal cores. The time needed to remove the internal core from the metal part is significantly reduced which reduces cost and increased productivity.
These sulfur-containing organic binder compositions and/or foundry mixes are preferred for the cold-box process using phenolic urethane binders. Their advantages are most apparent when used to make internal cores that will be used in a casting assembly to cast aluminum parts.
This invention can be used in any process, which utilizes an organic binder for making foundry shapes. For purposes of describing this invention, a foundry shape is any shape made from a foundry aggregate and organic binder that is used in a molding assembly for casting metal parts. A casting assembly is an arrangement of foundry shapes in a pattern such that a metal casting will be produced when molten metal is poured into the casting assembly and allowed to cool. An internal core is a core that is imbedded in the casting assembly. A foundry mix is a mixture of a foundry binder, and aggregate, and possibly a curing catalyst.
Foundry shapes can be prepared according to this invention by: (a) a cold-box or no-bake process which involves curing the foundry shape with a catalyst; and (b) heat cured processes such as the hot-box or warm-box processes, or the shell process which involves curing a foundry shape prepared with a novolak resin, a foundry aggregate, and hexamethylene diamine.
Any no-bake or cold-box organic binder, which will sufficiently hold the foundry mix together in the shape of a foundry shape such as a core or mold and polymerize in the presence of a curing catalyst, will work. Examples of some of the better-known organic binders include phenolic urethanes; aqueous alkaline phenolic resole resins; acrylic/epoxy resins; furan resins; and phenolic shell resins based on novolak resins. These binders are well known in the art and many of them have more than one component. See for instance U.S. Pat. Nos. 3,485,797 and 3,409,579 which relate to phenolic urethane binders cured with an amine catalyst; U.S. Pat. No. 4,526,219 which relates to epoxy-acrylic resins cured with sulfur dioxide and an oxidizing agent; U.S. Pat. No. 4,985,489 which relates to alkaline phenolic resole resins cured with carbon dioxide; U.S. Pat. No. 4,750,716 which relates to alkaline phenolic resole resins cured with methyl esters, all which are hereby incorporated by reference. Particularly preferred are phenolic urethane binders known as ISOCURE(copyright) cold-box and no-bake binders sold by Ashland Chemical Company.
Any sulfur compound where the sulfur atom is chemically bonded by two covalent bonds can be used as the sulfur compound. Typically used, as the divalent sulfur compound, is elemental sulfur. However, other divalent sulfur compounds can be used to augment core removal. For example, vulcanization accelerators such as sulfides; disulfides; sulfonamides; thiocarbamates; thiurams; tetrasulfides; xanthates; thiadiazines; thioureas; dithiocarbamates and benzothiazolethiolates of zinc, lead, and other heavy metals; and the like can be used separately, or in combination, with elemental sulfur.
The action of sulfur to promote removal of organically bound cores can be enhanced with catalysts. For instance, nucleophilic, electrophilic, and free radical catalysts are know to cleave S-S bonds and create polysulfides that are more reactive than cyclooctasulfur. xe2x80x9cReactions of Sulfur with Organic Compoundsxe2x80x9d, Voronkov, M. G. et. al., Consultants Bureau of New York, a Division of Plenum Publishing New York, N.Y. (1987, pages 40-45, QD 412.S1R37 ISBN 0-303-10978-6). The action of sulfur can also be enhanced with phosphines, organomagnesium compounds, group IVA organometallic compounds, amines, sulfides, polysulfides, mineral and organic protic acids, Lewis acids such as boron and aluminum compounds, copper, zinc, platinum, activated charcoal, zinc sulfide, fluorides, metal oxides. Free radical generators such as peroxides, persulfates, azo compounds, and the like could also augment the action of sulfur.
The sulfur is added to the foundry aggregate and/or one or more of the components of the organic binder, where the component may be reactive or non reactive. As was mentioned before, one of the preferred organic binders is a phenolic urethane binder cured by an amine catalyst. This binder comprises a phenolic resin component or and organic polyisocyanate component, both of which are reactive components. The divalent sulfur compound can be added to either of these components.
The divalent sulfur compound can also be dissolved or dispersed in a liquid or solid medium before it is added to the foundry aggregate or the organic binder. Examples of liquid media that can be used to dissolve the divalent sulfur compound are solvents, e.g. carbon disulfide, petroleum distillate, or a resin such as a phenolic resin.
The divalent sulfur compound can also be dispersed in a dry medium such as a foundry additive mixture as disclosed in U.S. Pat. No. 4,735,973 and the like. Although a dispersant may be used in some situations to improve the dispersability of the divalent sulfur compound, it is not required to practice this invention. The sulfur is preferably dispersed in the binder in an amount of from 10 to 100 parts based on the weight of the binder.
The amount of sulfur needed to facilitate core removal and/or shakeout is from 1 to 200 weight percent based on the weight of the organic binder, preferably from 10 to 100 weight percent; or from 0.1 to 2.0 weight percent based on the weight of the foundry aggregate, preferably from 0.3 to 1.0 weight percent.
As was mentioned previously, curing the foundry shape preferably takes place by the cold-box process. The cold-box process involves blowing or ramming the foundry mix into a pattern where it is shaped, and then curing the foundry shape with a vaporous or gaseous catalyst. Various vapor or vapor/gas mixtures or gases such as tertiary amines, carbon dioxide, methyl formate, and sulfur dioxide can be used depending on the chemical binder chosen. Those skilled in the art will know which gaseous curing agent is appropriate for the binder used. For instance, epoxy-acrylic binders cured with sulfur dioxide in the presence of an oxidizing agent are described in U.S. Pat. No. 4,526,219, which is hereby incorporated into this disclosure by reference. Alkaline phenolic resole resins cured with carbon dioxide are described in U.S. Pat. No. 4,985,489, which is hereby incorporated into this disclosure by reference. Alkaline phenolic resole resins cured with methyl esters are described in U.S. Pat. No. 4,468,359, which is hereby incorporated into this disclosure by reference.
As was mentioned previously, the preferred cold-box binder is an ISOCURE(copyright) phenolic urethane binder cured by passing a tertiary amine gas, such a triethylamine, through the molded foundry shape in the manner as described in U.S. Pat. Nos. 3,409,579 and 3,485,497, which are hereby incorporated into this disclosure by reference. These binders are based on a two-part system, one part being a phenolic resin component and the other part being a polyisocyanate component. Typical gassing times are from 0.5 to 3.0 seconds, preferably from 0.5 to 2.0 seconds. Purge times are from 1.0 to 60 seconds, preferably from 1.0 to 10 seconds.
The phenolic resole resin used in the phenolic resin component is preferably prepared by reacting an excess of an aldehyde with a phenol in the presence of either an alkaline catalyst or a metal catalyst. The preferred phenolic resins used in the subject binder compositions are well known in the art, and are specifically described in U.S. Pat. No. 3,485,797 which is hereby incorporated by reference. These resins, known as benzylic ether phenolic resole resins are the reaction products of an aldehyde with a phenol. They contain a preponderance of bridges joining the phenolic nuclei of the polymer, which are ortho-ortho benzylic ether bridges. They are prepared by reacting an aldehyde and a phenol in a mole ratio of aldehyde to phenol of at least 1:1 in the presence of a metal ion catalyst, preferably a divalent metal ion such as zinc, lead, manganese, copper, tin, magnesium, cobalt, calcium, and barium.
The phenols use to prepare the phenolic resole resins include any one or more of the phenols which have heretofore been employed in the formation of phenolic resins and which are not substituted at either the two ortho-positions or at one ortho-position and the para-position such as unsubstituted positions being necessary for the polymerization reaction. The aldehyde used to react with the phenol has the formula RCHO wherein R is a hydrogen or hydrocarbon radical of 1 to 8 carbon atoms. The most preferred aldehyde is formaldehyde.
Organic polyisocyanates used in the organic polyisocyanate component are liquid polyisocyanates having a functionality of two or more, preferably 2 to 5. They may be aliphatic, cycloaliphatic, aromatic, or a hybrid polyisocyanate. Mixtures of such polyisocyanates may be used. The polyisocyanates should have a viscosity of about 100 to about 1,000, preferably about 200 to about 600.
Solvents are typically used in the organic polyisocyanate component and/or phenolic resin component. If solvents are used in either, those skilled in the art will know how to select them. Typical organic solvents include aromatic solvents, esters, ethers, or possibly mixtures of these solvents. In general, the solvent concentration in the polyisocyanate and/or phenolic resin component is up to 80% by weight of the polyisocyanate or phenolic resin, typically in the range of 20% to 80%.
The phenolic resin and polyisocyanate are used in sufficient concentrations to cure in the presence of the volatile amine curing catalyst. In general the isocyanate ratio of the polyisocyanate to the hydroxyl of the phenolic resin component (NCO/OH ratio), is from 1.25:1.0 to 0.60:1.0, preferably about 0.9:1.0 to 1.1:1.0, and most preferably about 1.0:1:0.
Various types of aggregate and amounts of binder are used to prepare foundry mixes by methods well known in the art. The preferred aggregate employed for preparing foundry mixes is sand wherein at least about 70 weight percent, and preferably at least about 85 weight percent, of the sand is silica. Other suitable aggregate materials for ordinary foundry shapes include zircon, olivine, aluminosilicate, chrornite sand, and the like.
The amount of binder needed is an amount that is effective in producing a foundry shape that can be handled or is self-supporting after curing. In ordinary sand type foundry applications, the amount of binder is generally no greater than about 10% by weight and frequently within the range of about 0.5% to about 7% by weight based upon the weight of the aggregate. Most often, the binder content for ordinary sand foundry shapes ranges from about 0.6% to about 5% by weight based upon the weight of the aggregate in ordinary sand-type foundry shapes.
Optional ingredients for the binder include release agents, benchlife extenders, and adhesion promoters to improve humidity resistance, e.g. silanes as described in U.S. Pat. No. 4,540,724 which is hereby incorporated by reference.
A description of the components used in the examples follows: